A magnetic flux leakage inspection method using magnetism is one of methods that have been used for inspecting a defect of a steel material. The magnetic flux leakage inspection method magnetizes a measured target by direct or alternating magnetization and detects a magnetic flux that leaks from a surface of the measured target mainly using a search coil as a magnetic sensor. Recently, a magnetic resistor device (MR), a hall device, a magnetic impedance device (MI), and the like have begun to be used as the magnetic sensor. In the magnetic flux leakage inspection method using magnetism, when the steel product as the measured target is magnetized by the direct or alternating magnetization, if the measurement target has a defect on a surface, a leakage magnetic flux leakage is generated on the surface. Thus, the magnetic sensor measures a magnetic field component parallel to a surface of the leakage magnetic flux or a magnetic field component perpendicular to the surface of the leakage magnetic flux. A method of magnetizing the measured target includes the direct magnetization and the alternating magnetization using exciting coils. The direct magnetization can uniformly magnetize a cross section of the measured target and thus is mainly used for detecting a defect inside. The alternating magnetization exerts a frequency-dependent skin effect and thus is mainly used for the inspection near the surface of a material. A magnetic sensor with a sensitivity covering a low frequency wave such as the MR, the hall device, and the MI device allows excitation with the low frequency wave and thus is used for detecting a defect deep inside the measured target. The magnetic flux leakage inspection method is generally included in an eddy-current inspection method. The eddy-current inspection method and the magnetic flux leakage inspection method are inspection methods with the same configuration in which the magnetic field is applied to the measured target and the measurement is performed with the magnetic sensor. An eddy-current inspection method applying a low frequency magnetic field and detecting a magnetic field component parallel to a surface of a sample is described in Japanese Patent No. 3987941 (Patent document 1).
The measured target takes various shapes. For detecting a defect of a cylindrical steel pipe and steel bar, it is a common practice to insert the measured target through an encircling coil to be subjected to the alternating magnetization. To inspect a large steel pipe, tank, and the like, since the surface is approximately flat, it is a common practice to attach the exciting coils to a U-shaped yoke member and bring both end portions of the yoke member in contact with the measured target to perform alternating magnetization on the measured target (see, Non-patent documents 1 and 2).
Multipoint measurement data obtained by measuring the measured target at various positions with such a magnetic flux leakage inspection has conventionally been processed as follows. Specifically, an output from the magnetic sensor is directly used as a signal intensity, and a position and a size of a defect are estimated with reference to a change of the signal intensity at measurement positions. Recently, the output from the magnetic sensor has been detected by a lock-in amplifier in synchronization with the exciting coils to analyze a phase in addition to the signal intensity. Specifically, a change of the phase in addition to the change of the signal intensity at the measurement points are graphed to estimate a position and a size of the defect. For example, Japanese Unexamined Patent Application Publication No. 2007-64628 (Patent document 2) describes an analysis method for identifying a depth and a size of a defect from a graph showing a change of a signal intensity.